The present invention relates to magnetic resonance imaging (MRI) and, in particular, to radio frequency (RF) coils.
MRI has been widely used by radiologists as a powerful diagnostic tool. MRI techniques have several advantages including excellent soft tissue viewing and angiography. Neurovascular MRI has become a standard diagnostic procedure in the hospitals. Frequently, a neurovascular MRI protocol requires imaging coverage from the Circle of Willis to the aortic arch with a field-of-view (FOV) of about 36 cm. Another imaging protocol with a smaller FOV for high resolution carotid imaging is of great interest also. Finally, it would be a plus if the same neurovascular coil could be used for head/neck imaging. An ideal neurovascular RF coil would provide multiple imaging modes with different FOV, good S/N, good uniformity, and a patient friendly structure.
The concept of a co-planar array (Roemer et al., U.S. Pat. No. 4,825,162) has been widely used in neurovascular coils for horizontal field systems. Typically, partially overlapped surface coils are placed in a co-planar fashion to extend the coverage as required by neurovascular imaging protocols. Multi-mode imaging is achieved by activating different combinations of coil elements. Coil isolation between neighboring elements is achieved through the well known overlapping technique to cancel the mutual inductance. Coil isolation between elements other than immediate neighbors is achieved through the application of low noise amplifiers (LNA) in the resonance circuits.
The principle of MRI involves exciting protons and detecting their free induction decay signals. Each proton possesses a tiny magnetic moment precessing about the static magnetic field. The macroscopic behavior of millions of protons can be represented by a resultant magnetization vector aligning with the static magnetic field B0. A strong RF excitation pulse effectively tips the magnetization away from B0. The free induction decay of this magnetization is detected in a plane perpendicular to B0. Thus the normal direction of an receive RF coil must be perpendicular to the direction of the static magnetic field B0 for maximal signal induction.
As a result, co-planar array coils are effective for horizontal MRI systems. However, such co-planar surface array coils are, in general, inefficient for a vertical field system because the condition required for maximal signal detection can hardly be fulfilled. Various modifications to the co-planar designs have been proposed with limited success.
A more effective array configuration is needed for a vertical field neurovascular coil to provide good S/N with multi-imaging modes.